1. Field of the Invention
The present invention relates to electrophotographic printers, and, more particularly, to fusers in electrophotographic printers.
2. Description of the Related Art
In an electrophotographic (EP) printer, unfused toner particles are electrostatically attracted to the media to form an image. In order for the image to be fixed permanently the media must be fused. By combination of high temperature and pressure the toner is melted and forced to adhere to the media.
In color printing the fusing requirement is more stringent than merely ensuring that the toner adheres to the media. As there are multiple layers of toner, more energy is required to fuse the toner. Sufficient energy must be added to the media and toner such that the toner becomes transparent. The ability to mix colors and the ability to produce good quality transparencies depends on the ability to make the toner transparent. The trend in current printer technology has been to attempt to reduce standby power requirements and reduce warm up times for the fuser. For this reason a belt and a ceramic heater fuser is highly desirable. Due to the low thermal mass of such a fuser it has a very short warm up time, and no standby mode is required. This type of fuser has been used in several monochromatic printing applications. However, this type of fuser has not been successfully implemented for a color printing application. Color fusers have to fuse a much higher toner mass/area ratio. The higher coverage presents two challenges; first that all the layers of toners must be adequately fused and second that the fused toner must release cleanly from the belt surface. Color prints and especially transparencies are more sensitive to print quality defects than mono prints. Most color fusers therefore are compliant hot roll fusers, which are expensive and slow to warm up.
The type of belt used for mono fusers typically has a polyimide layer of 50-60 microns and a top coat of either spray coated or dip coated polytetraflouroethylene (PTFE), perfluoroalkoxy (PFA), or polytetrafluoroethylene-perfluoromethyl vinyl ether (MFA). MFA is the trade name for a modified PFA made by Ausimont USA Inc. (now Solvay Solexis), which has lower molecular weight and also a lower melting point than PFA. The coating of PTFE does not release cleanly at the level of mass/area required for color printing. Normally in a hot roll fuser a PFA sleeve is used to get the necessary release properties. Also the polymide belt lacks compliance and thus cannot conform well to the toner pile height variation and there is no xe2x80x9ckneadingxe2x80x9d of the toner as there would be with a compliant hot roll. A belt with a rubber layer and a PFA sleeve may be adequate to fuse color prints but such a belt is difficult to manufacture and expensive. Moreover by adding the rubber layer one negates the fast warm up time advantage for the belt fuser. The belt fuser has yet another limitation in that the nip pressure tends to be low compared to hot roll fusers. Since the belt has to slide relative to the heater housing the friction between the belt and other components must be minimized. Thus the force applied to the nip is limited also.
A color fuser may include a pressure roller with a heater positioned within the roller. The heater thus heats the roller from the inside out. Since the outer periphery of the pressure roller is used during the fusing process, the warm up time of the roller is the time necessary for the roller to transfer heat to the outside periphery of the roller where work is performed. This results in a slow warm up time of the color fuser.
What is needed in the art is an electrophotographic printer with a fuser having both a fast warm up time and sufficient thermal mass to fuse color images.
The present invention provides a fuser which externally heats a pressure roller through conduction heat transfer from a heater positioned within an endless belt.
The invention comprises, in one form thereof, a fuser for fusing an image to a toner side of a print medium in an electrophotographic printer. The fuser includes an endless belt defining an inner loop. A heater is positioned in contact with the belt within the inner loop. A pressure roller defines a nip with the belt and is positioned adjacent the toner side of the print medium.
The invention comprises, in another form thereof, a method of fusing toner particles to a print medium in an electrophotographic printer, including the steps of: energizing a heater within an inner loop of an endless belt; transferring heat via conduction from the heater, through the belt, and to a pressure roller against the belt; transporting the print medium through a nip defined between the belt and the pressure roller such that the toner particles are in contact with the pressure roller; and fusing the toner particles to the print medium using heat from the pressure roller.
An advantage of the present invention is that the pressure roller is heated externally (from the outside periphery) to reduce warm up times.
Another advantage is that the pressure roller has a higher thermal mass to fuse images with a higher toner density.
Yet another advantage is that the pressure roller has good compliance and release properties using a multi-layer configuration.
A further advantage is that the endless belt has a lower thermal mass to heat up quickly.
Still another advantage is that the thin belt allows heat to be transferred effectively and quickly via conduction to the pressure roller.